In many aircraft, the environmental control system, ECS, is powered from engine "bleed air", i.e. air extracted from bleed-ports located at the intermediate and last stages of the aircraft engine compressors. This is premium-air inasmuch as it impacts very unfavorably on the performance of the engine, and results in thrust losses and fuel penalties on the engine. In the guest for fuel-efficient air-transports, it is essential that the method of extracting power for the air conditioning system, be optimized from a power and fuel consumption point of view.
It is of importance and relevant to short-haul and short-to-mediumn range aircraft that the same quest for fuel-conservation in these aircraft could lead to the selection of turbo-prop engines. These engines, however, have lower "core-flows" than the jet engines and, as a consequence, the amount of air available for cabin air conditioning is usually more limited. At the same time, these aircraft also have a high passenger-density-to-aircraft-volume and therefore require good air ventilations rates. On the other hand, many wide-bodied aircraft (which have lower passenger/volume densities), often dump large amounts of cabin air overboard, in order to take on adequate quantities of fresh air. This also reflects as a significant weight and fuel-penalty.
One prior art method of providing power to aircraft ECS systems is to utilize the aircraft engines to mechanically drive the cabin compressors, usually via a gearbox arrangement between the engine and the compressor. Systems such as this are exemplified in U.S. Pat. Nos. 2,614,815 to Marchant et al, 2,585,570 to Messinger et al, 2,678,542 to Stanton, and 2,697,917 to Mayer.
While not using bleed air, these systems often involve nevertheless, use of complex gear drives, placement of compressors in hostile wing environments, and less than ideal fuel consumption characteristics.
Yet another method of driving cabin compressors is disclosed in copending U.S. patent applications U.S. Ser. No. 181,079, filed Sept. 2, l980, now abandoned for "Direct-Driven Generator System for Environmental Control System and Engine Starting", and U.S. Ser. No. 183,609, filed Sept. 2, 1980, for "All-Electric Environmental Control System For Advanced Transport Aircraft", both assigned to the assignee herein. Both of these applications describe various types of ECS systems which are operated by an electric motor driven compressor. These systems afford enhanced fuel conservation and several other advantages over the bleed air and engine-driven systems discussed hereinabove, and as such are quite desirable.
Several other approaches to enhancing the efficiency of ECS systems have also been attempted in the prior art. One such approah is disclosed in U.S. Pat. No. 3,711,044 to Matulich. The Matulich patent teaches utilizing an auxiliary gas turbine power unit to reduce the fuel demand of a conventional bleed air ECS by varying the speed of the compressor which supplies pressurized air to the ECS. This approach to increasing the efficiency of the ECS system is based upon a complex control system for the integration of the ECS and the auxiliary power unit (APU) to provide partial control of the APU in response to ECS requirements.
Another group of prior art patents teach the utilization, at least to some extent, of aircraft cabin discharge air within an ECS system ostensibly designed for greater efficiency. U.S. Pat. No. 2,479,991 to Wood, for example, discloses a primary cabin compressor driven by an APU which is mechanically driven, via an overrunning clutch, by two turbines. Air from the primary compressor is cooled by expansion through a first turbine, which in turn, mechanically unloads the primary compressor via the overrunning clutch. In addition, cabin air is discharged through a second turbine which also augments the compressor via the overruning clutch. This system involves mechanical energy feedback, and is complex from a control standpoint. In addition, the system can only input mechanical energy back to the compressor when the turbines can drive through the "free running" clutch.
Other approaches to using cabin discharge air to increase ECS efficiency can be found in U.S. Pat. Nos. 2,491,462 to Wood, 2,851,254 to Messinger et al, 2,777,301 to Kuhn, and 4,091,613 to Young. All of these patents to some extent, utilize gas turbine power units (GTPU) interfaced with ECS systems. These systems utilize several types of rotational elements, combustors and the like to superheat and expand cabin discharge air and put it back into the ECS. Such systems are typically complex and expensive, and more importantly, are less than ideally fuel efficient in that they use additional energy in the form of gas turbine fuel.
Yet another approach to utilizing cabin discharge air can be found in U.S. Pat. No. 3,369,777 to Furlong. Furlong teaches using cabin discharge air to drive an air turbine, which in turn drives a suction fan that draws air through the double wall of the cabin. The discharge air also drives a compressor that recompresses the discharge air before it is discharged overboard. The compressor is utilized to load the turbine. While this system recovers some of the energy of the cabin discharge air, it nevertheless is less than ideally efficient vis-a-vis fuel comsumption and aircraft ECS operation.
While all of the systems of the above prior art United States patents and copending applications, incorporated by reference herein, are directed to obtaining various levels of efficiency in aircraft ECS and airflow systems, it is, nevertheless, desirable that aircraft ECS systems be optimized in terms of efficiency in view of the present day strong need for fuel-efficient air transports. It is essential, then, that the energy and fuel consumption in extracting power for aircraft ECS be optimized (minimized), whether that power is extracted via engine "bleed air", "mechanical" or "electrical" power systems.
Thus, it is a primary object of this invention to provide a method and system for optimizing aircraft ECS systems in terms of energy, and thus, fuel consumption.
It is another object of this invention to provide an "energy utilization" or fresh-air "make-up" system for optimizing "bleed-air", "mechanical", and "electrical" ECS systems in aircraft.
It is yet another object of this invention to provide a system for pressurizing-the-inlet of electrically or mechanically driven ECS cabin compressors to optimize such ECS systems in terms of fuel consumption.
It is another object of the present invention to provide a system for feeding-back power to an electric motor which drives a cabin compressor in an ECS, to reduce the load on the motor and thereby conserve energy.